I went for Mass Effect because I've never played Dragon Age, am not a Baldur's gate fan and find it hard to call KotoR their best franchise when the best game in that series wasn't made by Bioware.
There are lots of things wrong with the Mass Effect franchise, there isn't a game in that series without gaping chasms in terms of flaws and both the ring conversation mechanic and the relationship mechanic need to die right now or be seriously overhauled.
But it also shows off the absolute brilliant best of Bioware, a whole new creative universe, interesting species, fun little side characters new landscapes. It brings about a sense of wonder that can let you forget the awful AI and sidequests because darn it, everything in these games are why speculative fiction is so good